System and method for quantifying X-ray backscatter system performance

ABSTRACT

A system for quantifying x-ray backscatter system performance may include a support; a plurality of rods mounted on the support; the rods of the plurality of rods arranged parallel to each other, having generally curved outer surfaces, and being arranged in groups of varying widths, each group of the groups having at least two of the rods of a same width; and a user interface configured to be connected to receive a backscatter signal from an x-ray backscatter detector associated with an x-ray tube, apply a transfer function to generate a transfer curve representing x-ray backscatter for each rod of the plurality of rods from x-rays transmitted by the x-ray tube.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with United States Government support underContract No. N00019-11-G-0001 awarded by the Department of Defense. TheUnited States Government has certain rights in this invention.

TECHNICAL FIELD

The disclosure relates to devices for measuring the performance of x-rayimaging systems and, more particularly, to devices and methods forquantifying the performance of x-ray backscatter imaging systems.

BACKGROUND

It is often necessary to inspect structural components of large objects,such as aircraft, maritime vessels, automobiles, and other largeinvestment assemblies, for defects and damage. Other structural objectsthat may require inspection include petrochemical facilities, powergeneration facilities, nuclear facilities, water treatment facilities,and the like.

Inspection of such structures and facilities by partial or completedisassembly of the structures to visually inspect internal components ofinterest may be impracticable because it is too time-consuming andexpensive. In some instances it may not be possible to inspect thedesired component without partially destroying the component ofinterest. Further, transportation of such structures and components oflarge object or facilities to an inspection location may be difficult,expensive, and in some instances impossible.

A technique for inspecting such components is the use of x-ray imaging.Inspection by x-ray imaging requires an x-ray transmitter that transmitsx-rays sufficiently powerful to pass through the object of interest andits surrounding components to be detected by a film or other detectionmeans. Such devices are large, cumbersome and relatively expensive.

X-ray backscattering systems provide an inspection process in whichx-rays are reflected from the object or component of interest andrecorded by a detector or detectors. X-ray backscattering systems do notneed to be powerful enough to transmit x-rays entirely through thecomponent of interest and its surrounding components. Rather, partialpenetration is all that is required. X-ray backscatter inspectionsystems are smaller, more portable, and less expensive than traditionalx-ray imaging systems.

A problem with x-ray backscattering systems is that the referencestandard used for calibrating or adjusting traditionalthrough-transmission film or digital x-rays does not work for x-raybackscatter systems. Scattered x-rays off of the standards or gaugesused for traditional through-transmission x-ray inspection do notprovide relevant spatial frequency information because x-rays arescattered differently (i.e., with respect to intensity and angle) fromdifferent types of materials. Accordingly, there is a need for a methodand system for quantifying x-ray backscatter system performance.

Further, in order to image very small features or detect small cracks,such as stress corrosion cracks, the size of the openings in theaperture of an x-ray backscatter device must be very small, on the orderof 0.25 millimeters (mm) or 0.5 mm in diameter. However, with such smallapertures, the amount of x-ray flux that is emitted from the x-ray tubeis very low, which requires a relatively slow x-ray tube scan speed anda relatively small scan area for each raster scan, so that many moreraster scans are required for a given inspection area.

Often, x-ray tubes with relatively larger apertures may be used forgeneral scanning because they permit higher x-ray flux, which enables arelatively larger area to be scanned for each pass, and at a faster scanrate. However, switching between large and small apertures takes timeand requires multiple sets of apertures of different sizes. Anotherdisadvantage is that smaller apertures are more difficult and expensiveto fabricate than larger apertures. The dimensions of small aperturesmake reproducibility of apertures difficult and require softwarenormalization to achieve consistent flux intensities from the x-raytube.

SUMMARY

The disclosure describes a system and method for quantifying x-raybackscatter system performance that overcomes the disadvantages of priorsystems for testing x-ray backscatter system performance, especiallyprior test systems adapted from calibrating or adjusting traditionalthrough-transmission film or digital x-rays. The disclosed system ishighly portable and therefore can be transported to the site of theapparatus under test. In one embodiment, a system for quantifying x-raybackscatter system performance includes a support; a plurality of rodsmounted on the support; the rods of the plurality of rods arrangedparallel to each other, having generally curved outer surfaces, andbeing arranged in groups of varying widths, each group of the groupshaving at least two of the rods of a same width; and a user interfaceconfigured to be connected to receive a backscatter signal from an x-raybackscatter detector associated with an x-ray tube, apply a transferfunction to generate a transfer curve representing x-ray backscatter foreach rod of the plurality of rods from x-rays transmitted by the x-raytube.

In another embodiment, an x-ray backscatter system includes an x-raytube for emitting an x-ray field; an enclosure surrounding the x-raytube blocking the x-ray field; and the enclosure having an aperturepositioned to allow x-rays to exit the enclosure in a relatively narrowx-ray beam, the aperture being adjustable in one or both of preselectedsize of opening and preselected shape, whereby an amount of x-ray fluxthrough the aperture is varied.

Other objects and advantages of the disclosed system and method forquantifying x-ray backscatter system performance will be apparent fromthe following description, the accompanying drawings, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of the system for quantifyingx-ray backscatter system performance;

FIG. 2 is a front elevation of the modulation transfer function standardof the system of FIG. 1;

FIG. 3 is a display of a backscatter image of the modulation transferfunction standard of FIG. 2;

FIG. 4 is a display of a second backscatter image of the modulationtransfer function standard of FIG. 2;

FIG. 5 is a display of a backscatter image of the modulation transferfunction standard from two detectors of the system of FIG. 1, utilizinga graphical user interface of the system of FIG. 1;

FIG. 6 is a display of the line profile and peak-to-peak data displayedon the graphical user interface of FIG. 5;

FIG. 7 is a display of the modulation transfer function plot of the lineprofile of the display of FIG. 6;

FIG. 8 is a flowchart of the method for quantifying x-ray backscattersystem performance performed by the system of FIG. 1;

FIGS. 9A and 9B show side elevation in section and end elevation,respectively, of an aperture wheel having the disclosed adjustableapertures;

FIG. 10 is a side elevation in section of another embodiment of anaperture wheel having the disclosed adjustable apertures;

FIG. 11 is a detail showing an embodiment of the disclosed adjustableaperture;

FIG. 12 is a detail showing another embodiment of the disclosedadjustable aperture;

FIGS. 13A and 13B show a plan view and a side elevation, respectively,of the adjustable aperture of FIG. 12 mounted on an end of a spoke ofthe aperture wheel of FIGS. 9A and 9B;

FIGS. 14A and 14B show a plan view and a side elevation, respectively,of another mounting arrangement of the adjustable aperture of FIG. 12,mounted on the end of a spoke of the aperture wheel of FIGS. 9A and 9B;

FIGS. 15A, 15B, 15C, and 15D show plan views of another embodiment ofthe disclosed adjustable aperture mounted in an end of a squarecollimator;

FIGS. 16A, 16B, 16C, and 16D show plan views of another embodiment ofthe disclosed adjustable aperture mounted in an end of a roundcollimator;

FIG. 17 shows a plan view of another embodiment of the disclosedadjustable aperture, in which nano motors position two sets of shutters;and

FIGS. 18A and 18B show plan views of another embodiment of the disclosedadjustable aperture, in which piezoelectric elements position two setsof shutters.

DETAILED DESCRIPTION

The system for quantifying x-ray backscatter system performance is shownin FIG. 1 and generally designated 10. The system 10 may include amodulation transfer function (“MTF”) standard 12 and a display 14 thatmay include a user interface, such as a graphical user interface (“GUI”)16. The display 14 may receive a signal from x-ray backscatter detectors18, 20, which in an embodiment may be solid state detectors, associatedwith an x-ray tube 22, which in an embodiment may be a small filamentmicro-focus x-ray tube. An x-ray tube 22 in the form of a small filamentmicro-focus x-ray tube may be preferable in some applications because asmall filament micro-focus x-ray tube may be lighter and smaller in sizethan the x-ray tube of a conventional x-ray backscatter system,possesses a relatively smaller x-ray field of view 24, and may berelatively low in radiation leaking. The x-ray tube 22 may be connectedto a source of electrical power 26 by power cables 28. In an embodiment,power cables 28 may also supply power to the detectors 18, 20 and/or todisplay 14. The display 14 may receive signals from the detectors overcables 30, 32, respectively.

As shown in FIGS. 1 and 2, the MTF standard 12 may include a support 34in the form of a frame 55, which in an embodiment may be made ofaluminum channel, and in a specific embodiment square aluminum channel.A plurality of rods, generally designated 36, may be mounted on thesupport 34 and arranged parallel to each other. The individual rods 56of the plurality of rods 36 may have generally curved outer surfaces,and may be arranged in groups 38, 40, 42, 44, 46, 48, 50, 52, 54 ofvarying widths. In an embodiment, the rods 56 may be arranged in groups38-54 in which the rods 56 have a common width. Thus, the five rods 56comprising group 38 may be of a common width, the five rods comprisinggroup 40 may have a common width that is different from the width of therods in group 38 (in an embodiment, the width may be less than the rodsof group 38), the five rods comprising group 42 may have a width that isdifferent from the widths of the rods in groups 40 and 38 (in anembodiment, the width may be less than the rods of group 40), the fiverods comprising group 44 may have a width that differs from the widthsof the rods in groups 42, 40 and 38 (in an embodiment, the width may beless than the width of the rods in group 42), and so on for theremaining groups of rods 46, 48, 50, 52, 54.

In embodiments, groups of rods 38-54 may have greater or fewer rods 56of a common diameter in each group than the five rods per group depictedin FIG. 2. In other embodiments, the numbers of rods 56 may vary fromgroup to group in the groups 38-54. In still other embodiments, thestandard 12 may include groups (not shown) of rods 36 in addition to thegroups 38-54 that may be thicker or wider than the rods of group 38,thinner or narrower than the rods of group 54, or both.

In an embodiment, the rods 36 may be arranged in parallel in a commonplane on the support 34. In an embodiment, the curved rods 36 may beround or circular in cross-section. In an embodiment, the rods 36 may bemade of a polymer or other hydrocarbon material, such as nylon 12. Useof rods 36 that are made of a polymer and that are round or circular incross-section may be preferable because the composition and shape mayprovide maximum backscattering from the MTF standard 12. As shown inFIG. 2, in an embodiment, the rods 56 of each of the groups 38-52 of theplurality of rods 36 may be spaced evenly from each other. In anembodiment, the rods 56 of each of the groups 38-54 may be spaced fromeach other a distance equal to, or approximately equal to, the width ofthe rods in each of the groups.

As shown in FIG. 2, the support 34 of the MTF standard 12 may include aframe 55 having a pair of opposing side rails 57, 58 and a pair ofopposing end rails 60, 62. In an embodiment, the frame 55 may begenerally rectangular, such that the side rails 57, 58 are generallyparallel to each other, and the end rails 60, 62 are parallel to eachother. The rods 56 may be mounted on the frame 55 to extend between thepair of opposing side rails 57, 58. In an embodiment, the rods 56 may besecured within the frame 55 by set screws 64 so that the rods may beproperly tensioned on the frame. In a particular embodiment, the spaces66-68 between each end rail 60, 62, respectively of the pair of opposingend rails and a next adjacent one of the rods 70, 72, respectively, isgreater than a spacing between the rods 36. This increased separationmay avoid backscatter caused by the end rails 60, 62.

Although the MTF standard 12 may be positioned at an angle relative tothe x-ray tube, it is preferable to position the MTF standard such thatthe support 34 positions the plurality of rods 36 perpendicular, orsubstantially perpendicular, to the x-ray field of view 24 transmittedby the x-ray tube 22. This may be effected by mounting the support 34 ona stand 69, which in embodiments may be adjustable to raise and lowerthe MTF standard 12 as well as pivot the standard relative to the x-raytube 22 and backscatter detectors 18, 20. It also may be preferable toarrange the rods 56 on the support 34 in a sequence progressing fromrods having a relatively large thickness, such as the rods of group 38,to progressively thinner rods in groupings 40-54, in which each of thenext grouping of rods has a thinner diameter than the rods to the rightof it (as shown in FIG. 2). Thus, the groups of rods 38-54 are arrangedon the support 34 in a sequence progressing from a relatively largethickness to a relatively small thickness.

The display 14 may be configured to be connected to receive abackscatter signal from x-ray backscatter detectors 18, 20 associatedwith the x-ray tube 22 and generate a display on GUI 16 representingphoton counts of x-ray backscatter for each rod 56 of the plurality ofrods 36 from x-rays transmitted by the x-ray tube 22.

As shown in FIG. 3, the user interface 16 may display a backscatterimage 74 that may be received from either one of the detectors 18, 20(FIG. 1). The x-ray backscatter images from the rods 56 of group 38(FIG. 2) are shown as group 38A, the backscatter from the rods of group40 are shown as grouping 40A, the rods of group 42 are shown as group42A, and so on for backscatter images 44A, 46A, 48A, 50A, 52A and 54A,which correspond to groups 44-54, respectively. The x-ray backscatterimages for the thicker rods, such as the rods of groups 38, 40, 42appear on the image 74 relatively sharply defined as groupings 38A, 40Aand 42A. In contrast, the groupings of the progressively thinner rods of44-54 (FIG. 2) are relatively less defined, and are faint and blurry,such as for groupings 44A, 46A, and 48A, whereas the images 50A, 52A and54A may appear quite faint. Thus, the MTF reference standard 12 providesa spectrum over which the sensitivity of the detectors 18, 20 may bemeasured. In an embodiment, the image of FIG. 3 may be stored in storage76 connected to display 14 (FIG. 1).

The image 74 may be stored in storage 76 as a reference image. As shownin FIG. 4, image 77 shows x-ray backscatter images of groups 38B, 40B,42B, 44B, 46B, 48B, 50B, 52B, and 54B corresponding to x-ray backscatterfrom rod groups 38-54, respectively of the MTF standard 12. Images38B-54B are fainter and less sharp than their counterpart images 38A-54Aof image 74. Backscatter images for groups 48B-54B may be barelyvisible, indicating a much lower photon count detected by detector 18and/or 20. This may indicate an out-of-adjustment or out of alignmentcondition for the x-ray tube 22 and detectors 18, 20 (FIG. 1).

As shown in FIG. 5, the GUI 16 (FIG. 1) of the display 14 may show ascreenshot of the x-ray backscatter image 78 of the reference standard12 that is comprised of scans from both detectors 18, 20. The imageshows the combined backscatter images of the group 38 of rods 36 (FIG.2), shown as group 38C, the combined images of group 40 shown as images40C, and so on for groups 42-56, shown as images 42C, 44C, 46C, 48C,50C, 52C, and 54C. Again, the images 38C-54C may be fainter and lesssharp than the reference images 38A-54A of display image 74 of FIG. 3.

As shown in FIG. 6, the user interface 16 may be configured to show ascreenshot that includes an image 80 of a line profile 82 representingphoton counts of the backscatter images of, for example, the image 74 inFIG. 3. The line profile 82 thus shows the photon counts for the x-raybackscatter from rods 36 of group 38 (FIG. 2) as the peaks 38D, thephoton counts for group 40 as peaks 40D, and so on for groups 42-54 aspeaks 42D, 44D, 46D, 48D, 50D, 52D, and 54D, respectively. The lineprofile 82 thus shows the ability of the detectors 18, 20 (either singlyor in combination) to distinguish the individual rods for the groupingsof rods of progressively decreasing thickness. From the graph lineprofile 82, the photon counts progressively decrease from the thickerrod groupings 38, 40, 42, 44 (FIG. 2) and become less distinct in thegroupings 46, 48, 50, 52, 54, as evidenced by their corresponding lineprofiles 38D-54D. Groups represented by curves 48D-54D may be consideredindistinguishable.

Also as shown in FIG. 6, the GUI 16 may include peak-to-peak text datain display 84, listing frequency values 86, MTF frequencies 88, MTFstandard deviations 90, and optionally, provide a reference number incolumn 92 for each value. The GUI 16 also may include virtual buttonsthat may be actuated by a mouse for switching from one image to another,such as button 94 for reverting the user interface, button 96 forfinding the peaks on the line profile 82, button 98 for selecting aregion of interest (“ROI”) of the curve of the entire x-ray backscatterimage of FIGS. 3-5, button 100 for saving the project in storage 76(FIG. 1), button 102 for loading a reference image (such as the image ofFIG. 3), and button 104 for saving the reference image.

The GUI 16 also may include a virtual button for adding an MTF 106,removing the MTF with button 108, marking the reference with button 110,unmarking the reference with button 112, adding an MTF point with button114 and exporting the data with button 116. Further, the image shown inFIG. 6 may be selected by selecting the virtual tab 118 marked “Profile”at the top of the page. In contrast, the tab marked “Raw Data” 120 maybe used to call up the images of FIGS. 3, 4 and 5.

As shown in FIG. 7, the interface 16 may include a display 120 that mayinclude a plot 122 of the current MTF displayed on the user interface ofdisplay 80 of FIG. 6. The plot 122 may be used to record, monitor andcompare x-ray backscatter performance of the x-ray system being studied.In an embodiment, the MTF may be calculated by subtracting from the highaverage peak the low average peak, then dividing the difference by thesum of the high average peak and the low average peak: [(high averagepeak)−(low average peak]/[(high average peak)+(low average peak)].

In an embodiment, the plot 122 may represent the MTF (frequency) on theY-axis plotted against the frequency on the X-axis, taken from display84 of the user interface shown in FIG. 6. The Y-axis is a scale from 1to 100, with the 100 value representing the ratio of bright and darksignals from rods (e.g. rods 38 in FIG. 1) having the greatestthickness, shown as sine wave 38D in FIG. 6. The other points on plot122 represent ratios for rods of other thicknesses, i.e., from sinewaves 40D-48D in FIG. 6. The X-axis represents the frequency in terms ofnumber of rods per unit width, so as the rods decrease in thickness, thefrequency of rods per unit width increases. This plot 122 may be used torecord, monitor, and compare x-ray backscatter system performance. In anembodiment, the display 120 also may be used to show a plot 123 of areference MTF on the user interface of display 80 of FIG. 6.

The reference MTF plot 123 may be kept in storage 76 (FIG. 1) ofreadings of the system 10 taken as initial readings when the system hasbeen adjusted and when all components are properly calibrated. As shownin FIG. 7, current MTF plot 122 shows that the system 10 has degradedslightly from the reference settings, because the curve shows slightlylower values at each frequency.

As shown in FIG. 8, the process for testing a x-ray backscatter system10 using the reference standard 12 and user interface 16 is shown at126. As shown in step 128, an everyday system MTF measurement is takenusing the reference standard 12 and user interface 16 (FIG. 1). Asindicated in block 130, the initial data is taken, and in step 132 theMTF data is captured from the reference sample, such as the image inFIG. 3. This information is stored in storage 76, as indicated at step134 as a data file and/or a text file. The MTF software is then loaded,as indicated at step 136 and the raw data tab 120 (FIG. 6) is selected,as indicated at step 138.

As indicated at step 140, an MTF scanned image, such as the image ofFIG. 4, may be selected, and, as indicated at step 142, the photoncounts may be stored in a data and/or text file.

As indicated at step 144, a region of interest (ROI) or line segment maybe selected to yield a line profile 82 of display 80 of FIG. 6. In thenext step, indicated at 146, the peaks of the line profile are found byactuating the find peaks button 96 on the display of FIG. 6. In step148, the ROI is selected and added to the MTF, as indicated at step 150.As indicated in step 152, the MTF 15 plotted, yielding a plot such asplot 122 of display 120 of FIG. 7.

As indicated at step 154, the reference is loaded and a comparison ismade between the reference plot and the plot from the selected scannedimage taken at step 140, as indicated at step 156. As indicated atdecision diamond 158, if the reference plot and the test plot aredifferent, then, as indicated at step 160 the system parameters areadjusted, or the system evaluated to determine the reason for disparity.In the alternative, as indicated at block 162, if the reference plot andthe project plot are the same; that is, they coincide, as shown in FIG.7, the test is completed.

The disclosed system and method for quantifying x-ray backscatterperformance solves the problem of the need for measuring, tracking andcomparing the imaging capability of x-ray backscatter systems. Thesystem and method may achieve this by their ability to resolve smallfeatures with adequate image contrast. Spatial frequency response of animaging system is the contrast at a given spatial frequency. Spatialfrequency may be measured in cycles or line pairs per millimeter(lp/mn), which is analogous to cycles per second (Hz) in audio systems.Lp/mn, or cycles per pixel (c/p) or line widths per picture height(LW/PH) can all be used. High spatial frequencies correspond to fineimage detail. The response of imaging system components tends to rolloff at high spatial frequencies (i.e., smaller features). High spatialfrequencies correspond to fine image detail. The more extended theresponse, the finer the detail and the sharper the image.

The disclosed system and method utilizes a special reference standard inthe form of the standard 12, and a modulation transfer function (“MTF”)that enables the performance of x-ray backscatter systems to bequantified, and thus compared against a reference standard. Variationsin backscatter x-ray intensity across the image of the standard arequantified and displayed on the user interface 16.

As shown in FIGS. 9A and 9B, the system 10 (see FIG. 1) may be utilizedwith an x-ray tube 200, in place of x-ray tube 22, having an aperture202 that may emit an x-ray field of view 204. The x-ray tube 200 may bepositioned within an enclosure in the form of aperture wheel 206 havinga generally cylindrical housing 208 that may enclose the x-ray tubepartially or completely. The aperture wheel 206 may have a plurality ofspokes 210, 212, 214, 216, 218 projecting radially outward from thehousing 208. Each of the spokes 210-218 may have a hollow interiorpassage 220, 222, 224, 226, 228, respectively. The hollow interiorpassages 220-228 may be positioned to be rotated into registration withan opening 230 in the housing 208 that allows a collimated x-ray beam232 from the x-ray tube 200 to exit the aperture wheel 206.

In an embodiment, each of the spokes 210-218 may include an adjustableaperture 234, 236, 238, 240, 242, respectively, mounted on an end of thespoke and in communication with the respective hollow interiors 220-228.As will be discussed, the adjustable apertures 234-242 may beselectively adjusted by an operator, or selectively adjustedautomatically, to vary the shape and/or size of the opening, which mayshape the collimated x-ray beam 232, as well as vary the flux orintensity of the x-ray beam 232 exiting the spokes 210-218. In anembodiment, only one of the apertures 234-242 may be adjustable, and theremaining apertures may be fixed; that is, the remaining apertures maybe fixed and non-adjustable in the size of their respective openings.

The aperture wheel 206 may include a motor drive 244 that may beconnected by a drive linkage 246 to rotate the spokes 210-218 to bringsuccessive spokes into registry with the opening 230 to perform a rasterscan of an object 247 (FIG. 10) to be inspected.

As shown in FIG. 10, in another embodiment, the x-ray tube 200 may belocated and wholly or partially enclosed within an enclosure in the formof an aperture wheel enclosure 248 having adjustable apertures 234, 236,238, 240, and 242. The adjustable apertures 234-242 may be mounteddirectly on the housing 250, rather than on the ends of spokes 210-218as shown in FIGS. 9A and 9B. The housing 248 may include openings 252,254, 256, 258, 260 that may be in registration with the adjustableapertures 234-242 mounted on the housing to allow a collimated x-raybeam 232 to exit the enclosure 248 from the interior 262 of theenclosure 248. In the embodiments of FIGS. 9A, 9B and 10, the x-ray beam232 may be collimated and result from the x-ray field 204 emanating fromthe x-ray tube 200. The enclosure 248 may take the form of a wheelsimilar in design and operation to the aperture wheel 208 of FIGS. 9Aand 9B.

The housing 250 may be driven by a motor 244 through a mechanicallinkage 246 to rotate to bring the adjustable apertures 234-242successively into alignment with the x-ray field of view 204, so thatcollimated x-ray beams 232 may successively emanate from the adjustableapertures 234-242. The motor 244 may be actuated to rotate the enclosure248 to perform raster scans 263 on the object 247. In embodiments, theobject 247 may be a body panel or a structural member of a vehicle 249,such as an aircraft, a spacecraft, a land vehicle, or a marine vehicle.In specific embodiments the object 247 may be a section of skin of afuselage or of a wing of a vehicle 249 such as an aircraft, and mayinclude rivets. The x-ray backscatter from the raster scans 263 may bemeasured by detectors 18, 20 (see FIG. 1) associated with the x-ray tube200 and stored in storage 76 and/or displayed on a GUI 16 of display 14.

As shown in FIG. 11, an exemplary adjustable aperture 264 may includetwo flat, plate-shaped shutters 266, 268 that may be positioned by amotor 270, which in embodiments may be a small electric motor or ananomotor. The motor 270 may be connected to drive, or include, athreaded shaft 272 that threads into a boss 274 that may be connected tothe shutter 268. The motor 270 may be mounted on a fixed support, suchas fixed shaft 276, that may be held in place by a boss 278 attached tothe fixed shutter 266, or in an embodiment, to adjacent fixed structuresuch as the aperture wheel 248 (FIG. 10), or a spoke 210-218 (FIG. 9A).The two shutters 266, 268 may slide against an aperture wall segment 280of the end of, for example, the spoke 210-218 of the enclosure 206 ofFIGS. 9A and 9B, or may be a portion of the enclosure 248 of FIG. 10.Alternatively, the adjustable aperture may be mounted on an end of x-raytube 22 (FIG. 1).

In one particular embodiment, each of the shutters 266, 268 may be inthe shape of a parallelogram. The shutters 266, 268 may be positioned inpartially overlapping relation to each other, having straight sides 282,284 that are oriented at an angle of approximately 60° relative to eachother, and at an angle of approximately 60° to the adjacent straightside 286 of the segment 280 of the housing 208, 248 of the aperturewheel 206, 248. As a result of this orientation, the opening 288 formedby the edges 282, 284, 286 may maintain the same shape, which in anembodiment may be an equilateral triangle, even as the shutter 268 ismoved to the left or right as indicated by the arrow A in FIG. 11relative to shutter 266 and to wall segment 280.

Movement of the shutter 268 to the left in FIG. 11 in the direction ofarrow A by the motor 270 may cause the opening 288 to decrease in size,but maintain its shape of an equilateral triangle. Conversely, movementof the shutter 268 to the right in the direction of arrow A by motor 270may cause the opening 288 to increase in size (i.e. cross-sectionalarea) while maintaining its shape, which in this embodiment is anequilateral triangle shape. Movement of the shutter 268 relative tofixed shutter 266 and wall segment 280 to vary the size of the opening288 in this fashion may result in increasing or decreasing the flux orintensity of the x-ray beam 232 emanating from the aperture wheel 208,248.

Actuation of the motor 270 to vary the size of the opening 288 may beeffected by a computer control 289, which in embodiments also mayactuate the motor 244 to cause the x-ray tube 200 and enclosure 206, 248to perform raster scans on the object 247. In embodiments, the control289 may actuate the motor 270 on the fly; that is, during a raster scans263 (FIG. 10), to vary the x-ray flux in the x-ray beam 232 directed atthe object 247 being inspected. In other embodiments, the x-ray tube 200and enclosure 206, 248 may be actuated by motor 244 to complete rasterscans 263 of an area on the object 247 at one aperture size, for examplean aperture 288 having an average width of 2 mm.

The control 289 then may actuate the motor 270 to adjust the size of theopening 288 of the adjustable aperture 264 to a smaller area, forexample an opening having an average width of 0.5 mm. The motor 244 thenmay actuate the aperture wheel 206, 248 to perform a second, subsequentraster scan 263 of that same area of the object 247 using more scanlines than the previous scan and at a lower x-ray flux or intensity,which may provide a higher resolution, higher contrast scan to bedetected by detectors 18, 20 (FIG. 1). In an embodiment, the control 289may actuate the motor 270 to decrease the average width of the opening288 further, for example to 0.25 mm, and the control 289 actuate themotor 244 to cause the x-ray tube 200 and enclosure 206, 248 to performa third, subsequent raster scan 263 of that same area of the object 247using more scan lines than the previous scan and at still lower an x-rayflux or intensity, which may provide an even higher resolution scan thanthe two previous scans.

This procedure may be desirable in a process wherein the first scan 263of the object 247 may find an anomaly on or in the object, which may befound by the detectors 18, 20 receiving the x-ray backscatter, and shownon GUI 16 of display 14. Then, the x-ray tube 200 may be directed bymotor 244 and configured by control 289 to perform subsequent scans 263of the anomaly on the object 247 at increasingly higher resolutions(i.e., using apertures 288 made successively smaller by motor 270). Thesuccessive scans 263 may be performed using a single adjustableaperture, such as the adjustable aperture 264 shown in FIG. 10, whichmay be adjusted during or after each raster scan 263. Alternatively, thesuccessive scans 263 may be performed using two or more multipleadjustable apertures 234-242 mounted on, for example, the spokes 210-218of aperture wheel 206, or aperture wheel 248, wherein the adjustableapertures may be adjusted on the fly to openings 288 of successivelysmaller areas.

As shown in FIG. 12, an adjustable aperture 290 may be configured tohave two flat, plate-shaped moving shutters 292, 294, each having aprojecting boss 296, 298 that is engaged with a respective threadedshaft 300, 302 operably connected to a motor 304, which may be ananomotor. In embodiments, the motor 310 may be fixed to the spoke210-218, or to the enclosure 248, or the shafts 302 may be reversethreaded and/or counter-rotated by the motor 304. The motor 304 may beactuated by an operator to selectively move the shutters 292, 294 in thedirection of arrow B to increase or decrease the size of the opening306.

In an embodiment, shutter 294 may include an edge 308 that is positionedat a 60° angle to the edge 310 of an adjacent portion of the wall 312 ofthe end of a spoke 210-242 (FIG. 9A), for example, and shutter 292 mayinclude an edge 312 that is oriented at a 60° angle to the edge 310, andwith respect to edge 308. Thus, edges 308, 310, 312 may meet to form anequilateral triangle-shaped opening 306. Shutters 292, 294 may overlapso that actuation of the shafts 300, 302 by the motor 304 may cause theshutters to variably overlap each other as they move toward or away fromeach other in the direction of arrow B, either toward the center of theadjustable aperture 290, or away from the center of the adjustableaperture. An advantage of the adjustable aperture 290 of FIG. 12 overthe adjustable aperture 272 of FIG. 11 is that the opening 306 remainscentered relative to the apertures 252-260 of the aperture wheel 248 ofFIG. 10, or with the hollow interiors 220-228 of the spokes 210-218 ofthe aperture wheel 208 of FIGS. 9A and 9B.

In a specific application of the adjustable aperture 264, as shown inFIGS. 13A and 13B, the aperture 264 may be mounted on an end 314 of aspoke 316, which may be one or more of the spokes 210-218 of FIGS. 9Aand 9B. The end cap 318 may be attached to the spoke 316 by a suitableadhesive, pressing it into the spoke, or by mechanical fasteners suchthat it is removable from the spoke. The spoke 316 may include an endcap 318 having an opening 320 in registration with the opening 288formed by the adjustable aperture 264. The end cap 318 may be shaped tocover and enclose the motor 270 and shafts 272, 276, and partially coverand enclose the shutters 266, 268. The opening 320 may be inregistration with the opening 288 so that the collimated x-ray beam 232may exit the spoke 316. The opening 320 may be shaped to equal or exceedthe largest size of the opening 306 of the adjustable aperture 264.

As shown in FIGS. 14A and 14B, a spoke 316 may include an end cap 319shaped to cover and enclose the adjustable aperture 290. The end cap 319may be attached to the spoke 316 by a suitable adhesive, pressing itinto the spoke, or by mechanical fasteners such that it is removablefrom the spoke. The end cap 319 may include an opening 321 shaped andpositioned to be in registration with the opening 306 formed by theadjustable aperture 290. The end cap 319 may partially cover not onlythe overlapping shutters 292, 294 but may completely cover the motor 304and shafts 300, 302. In contrast to the end cap 318 of FIGS. 13A and13B, where the opening 320 is offset from the center of the end cap toaccommodate the shifting position of the opening 288 as the shutter 268is moved in the direction of arrow A for adjustment of size, the opening321 may be positioned substantially midway in the center of the end cap290 because the opening 306 remains centrally located as the result ofsymmetrical movement of the overlapping shutters 292, 294 of theaperture 290.

As shown in FIGS. 15A, 15B, 15C, and 15D, in another embodiment, theadjustable aperture may include a pair of substantially flat,plate-shaped fixed shutters 322, 324 that may be pressed into the openend 332 of a square spoke 334. The shutters 322, 324 may be shaped andpositioned on the open end 332 to form a gap 326 between them that maybe filled partially with a flat shim 328, which also may be pressed intothe open end 332. The shim 328 may have a slit 330 formed therethrough.In an embodiment, the shutters 322, 324 and shim 326 may be pressed intothe open end 332 of a spoke 334 of, for example, an aperture wheel 206(see FIG. 9A). In embodiments, the slit 330 may be oblong and generallyrectangular, as shown in FIG. 15A. In other embodiments, the slit may beshorter, such as slit 336 of shim 338 in FIG. 15B, or almost square andrelatively small, such as slit 340 of shim 342 in FIG. 15C. In otherembodiments, the shutters 322, 324, and shims 326, 338, 342, 346 may beattached to the open end 332 by a suitable adhesive, by a mechanicalconnection, such as by screws, or by welding or brazing.

In yet another embodiment, the orientation of the slit 330, 336, 340 maybe vertical or substantially vertical, as shown in FIGS. 15A, 15B, and15C, or horizontal, as shown for aperture 344 of shim 346 in FIG. 15D,relative to the direction of raster scan 263. For inspections such asstress corrosion crack detection, there is a need for improvedresolution along the linear scan direction. Smaller circular aperturesmay help, but as the aperture becomes very small, the whole is difficultand expensive to machine accurately, and also may limit the x-ray fluxand therefore slow down a backscatter x-ray scan to a point where it maybe impracticable to perform.

By providing the shims 328, 338, 342, and 346 with apertures 330, 336,340, and 344, respectively, press fit into, or otherwise attached to,the ends 332 of spokes 334, the cost of manufacture is reduced and thedesign is simplified of the aperture wheel 206, 248. In addition, theslits 330, 336, 340, 344 may allow increased resolution, due to thenarrow slit, but an increased flux due to the relatively wider opening,so scan speeds may remain high enough to be practicable. While verticalresolution may be reduced slightly, horizontal resolution, which isimportant for crack detection when their orientation is generallypredictable, may be improved over prior art apertures having a uniformcross-section.

Similarly, as shown in FIGS. 16A, 16B, 16C, and 16D, the shutters 348,350 may be substantially flat and plate-shaped, having a generallysemi-circular shape. The shutters 348, 350 may be press-fitted into theopen end 352 of a circular spoke 354 of an aperture wheel, such as theaperture wheel 206 shown in FIG. 9A, or aperture wheel 248 of FIG. 10.Such shutters 348, 350 may be shaped to form a gap 356 when pressed intoor otherwise attached to the ends 352 of the spokes 354, and a shim 358may be fitted between the shutters 348, 350 within the gap. The shim 358may have an aperture 350 therethrough of a predetermined width andlength, preferably having a length that may be a multiple of thedimension of the width to allow increased resolution, similar to slit330 in FIG. 15A. Similarly, in FIG. 16B, shim 362 may have a slit 364,and in FIG. 16C, shim 366 may have a slit 368, and in FIG. 16D, shim 370may have a slit 372. The orientation of the slit 372 in FIG. 16D may beat an angle to facilitate detection of a stress corrosion crack of aknown or expected orientation.

As shown in FIG. 17, an adjustable aperture 374 may include an outerpair of substantially flat, plate-shaped shutters 376, 378 having bosses380, 382, respectively, threaded into a threaded shaft 384 that may berotated by a motor 386, which in embodiments may be a nanomotor. Theadjustable aperture 374 also may include an inner pair of substantiallyflat, plate-shaped shutters 388, 390 including bosses 392, 394,respectively, threaded into a shaft 396 that may be rotated by a motor398, which in embodiments also may be a nanomotor. The outer pair ofshutters 376, 378 and inner pair of shutters 388, 390 may be mounted onan end 399 of a spoke 316 (FIG. 14B) of an aperture wheel 206 (FIG. 9A),or mounted on aperture wheel 248. The shafts 384, 396 may be threadedsuch that, when rotated by the motors 386, 398, respectively, the outershutters 276, 378 and inner shutters 380, 382 each move toward eachother or away from each other. The motors 386, 398 likewise may beattached to the end 399 of the spoke 316.

The inner pair of shutters 388, 390 may be oriented at an angle relativeto the outer pair of shutters 376, 378, and in a particular embodimentmay be oriented at a 90° angle to the shutters 376, 378. The outer pairof shutters 376, 378 may be positioned adjacent the inner pair ofshutters 388, 390, and in an embodiment immediately behind them, to formwith them a slit 400 that may be defined by an edge 402 of shutter 388,an edge 404 of shutter 390, an edge 406 of shutter 376, and an edge 408of shutter 378. The motors 386, 398 may be actuated by a control 410,which also may actuate motor 244 (FIGS. 9A and 10) so that the motors386, 398 may displace the panels 376, 378 relative to each other, andpanels 388, 390 relative to each other, so that the shape and open areaof the aperture 400 may be varied as desired. Further, thisadjustability of the aperture opening may be effected during a scan, ifdesired. The adjustable aperture 374 may be mounted on an end of aspoke, such as one or more of the spokes 210-218 in FIG. 9A, or mountedon the aperture wheel 248 of FIG. 10. In another embodiment, a singleadjustable aperture 374 may be mounted on an end of x-ray tube 22 (FIG.1).

As shown in FIGS. 18A and 18B, in another embodiment, adjustableaperture 412 may include an outer pair of adjustable shutters 414, 416,and an inner pair of adjustable shutters 418, 420, which may be mountedon an end 399 of a spoke 316 of an aperture wheel 206 or the x-ray tube22, or mounted on aperture wheel 248. Shutters 414, 416 may be attachedto piezoelectric actuators 422, 424 that are connected to voltagesources 426, 428. The voltage sources 426, 428 may be actuated by acontrol 430 so that the spacing between the shutters 414, 416 may bevaried by actuating the piezoelectric actuators 422, 424. In anembodiment, a second pair of piezoelectric actuators 432, 434 may beattached to and positioned between shutters 418, 420. Piezoelectricactuators 432, 434 may be actuated by voltage sources 436, 438controlled by control 430. Thus, piezoelectric elements 422, 424 maycontrol the shape, size, and aspect ratio of the slit aperture 440,which may be defined by edge 442 of shutter 414, edge 444 of shutter416, edge 446 of shutter 418, and edge 448 of shutter 420.

As shown in FIG. 18B, actuation of piezoelectric elements 422, 424 bycontrol 430 may cause the shutters 414, 416 to move in the direction ofarrows C, thus increasing the width of the slit 440 of the aperture 412.Similarly, control 430 may actuate voltage sources 436, 438 (shown inFIG. 18A) to activate piezoelectric elements 432, 434 to move shutters418, 420 toward or away from each other to vary the shape of the slit440. This actuation of piezoelectric elements 422, 424, 432, 434 may bedone selectively and without removing the aperture 412 from the x-raytube 22 or aperture wheel 206, 248 on which it is mounted. Further, thecontrol 430 may be programmed to vary the shape (including the aspectratio) of the slit 440 in a pre-programmed fashion.

Thus, with the embodiment of FIGS. 18A and 18B, a slit type adjustableaperture 412 may be adjustable on the fly using piezoelectric materials.The piezoelectric materials may expand or contract based upon an appliedvoltage across selected crystal orientations. This structure andassociated method may be used for the adjustable apertures shown inFIGS. 9A-17 as well.

The adjustable apertures 234-242, 264, 290, 326, 338, 342, 346, 358,362, 366, 370, 374, and 412 shown in FIGS. 9A-18B preferably are made oftungsten. These adjustable apertures and associated method provide animproved set of adjustable apertures for an x-ray backscatter systemthat controls the size and shape of the x-ray beam that is rasteredacross the target structure 247 during a raster scan 263. Theseadjustable apertures may improve system performance by providing optionsbeyond the standard circular opening of selected size machined intotungsten. These apertures may be adjustable to reduce the need formultiple sets of apertures, each of a different, fixed size, and canprovide improved resolution of defects in selected directions withoutsacrificing flux or scan speed.

These disclosed adjustable apertures may be adjusted, either using themotors 270, 304, 386, 398, or by replacing the shims 328, 338, 342, 346,358, 362, 366, and 370 to provide an opening that may be very small, onthe order of 0.25 mm or 0.5 mm in width in order to detect small crackssuch as stress corrosion cracks. At the same time, and if desired, onthe fly, the adjustable apertures may be adjusted to have a largeraperture that allows for general scanning at higher flux levels, whichenables higher scan speeds. This can be done without replacing theentire aperture to achieve the desired size. The adjustable apertureseliminate the need for software normalization to achieve consistentx-ray backscatter flux intensity. Further, the orientation of theapertures may be selected such as the apertures 344, 372 in FIGS. 15Band 16B, or may be adjusted, such as the adjustable aperture 374 of FIG.17 and the adjustable aperture 412 of FIGS. 18A and 18B to provideimproved crack detection that results from orienting the wide dimensionof the aperture perpendicular aperture. Other orientations of theaperture relative to the predicted crack orientation may be selected toimprove the image resolution and defect detection.

While the methods and forms of apparatus herein described constitutepreferred embodiments of the disclosed system and method for quantifyingx-ray backscatter system performance, it is to be understood thatvariations may be made therein without departing from the scope of theinvention.

What is claimed is:
 1. An x-ray backscatter system, the systemcomprising: an x-ray tube for emitting an x-ray field; an enclosure thatis an aperture wheel that surrounds the x-ray tube, the enclosuredefining an end and an aperture mounted on the end of the enclosure,wherein the x-ray field exits the enclosure through the aperture; and aplurality of shutters that define an opening of the aperture, whereinone or more shutters of the plurality of shutters are moveable to varyat least one of a size and a shape of the opening, and wherein movementof the one or more shutters vary an amount of x-ray flux that exits theenclosure.
 2. The x-ray backscatter system of claim 1, wherein theaperture is selected from a variably overlapping stationary shutter andmovable shutter, a pair of variably overlapping movable shutters, and avariably adjustable slit aperture.
 3. The x-ray backscatter system ofclaim 2, wherein the adjustable slit aperture is adjustable by one orboth of mounting selected one of a plurality of shims of differentwidths and thicknesses on the aperture, and by selectively adjustinggaps between the plurality of shutters by motors.
 4. The x-raybackscatter system of claim 1, wherein the plurality of shutterscomprise a first pair of movable shutters and a second pair of moveableshutters disposed adjacent the first pair of shutters and oriented at a90 degree angle relative to the first pair of shutters.
 5. The x-raybackscatter system of claim 4, wherein the first pair of movableshutters are attached to a first piezoelectric actuator and the secondpair of movable shutters are attached to a second piezoelectricactuator, wherein the first piezoelectric actuator is connected to afirst voltage source and the second piezoelectric actuator is connectedto a second voltage source.
 6. The x-ray backscatter system of claim 5,further comprising a control system in communication with the first andsecond voltage sources for independently controlling the first voltagesource to move the first pair of shutters and for independentlycontrolling the second voltage source to move the second pair ofshutters to vary the size of the aperture.
 7. The x-ray backscattersystem of claim 1, wherein the enclosure further comprises a pluralityof spokes, wherein each of the plurality of spokes are spaced apartaround a periphery of the aperture wheel.
 8. The x-ray backscattersystem of claim 7, wherein each of a plurality of apertures are disposedat an end of each of the plurality of spokes.
 9. A method for producingx-ray backscatter, the method comprising: emitting an x-ray field usingan x-ray tube; surrounding the x-ray tube with an enclosure, wherein theenclosure defines an end and an aperture mounted on the end of theenclosure, and wherein the x-ray field exits the enclosure through theaperture; partially blocking the x-ray field with the aperture, whereinthe enclosure is provided in the shape of a wheel, and wherein eachaperture of a plurality of apertures are spaced apart along a peripheryof the wheel; and moving one or more shutters that are part of aplurality of shutters to vary an amount of x-ray flux through an openingof the aperture, wherein the one or more shutters move to vary at leastone of a size and a shape of the opening.
 10. The method of claim 9,further comprising rotating the enclosure in the shape of the wheel toperform a raster scan.
 11. The method of claim 10, wherein rotating theenclosure in the shape of the wheel to perform a raster scan furthercomprises aligning each of the plurality of apertures with the x-raytube.
 12. The method of claim 9, wherein varying the amount of x-rayflux through the aperture further comprises adjusting the aperture usingone of a variably overlapping stationary shutter and movable shutter, apair of variably overlapping movable shutters, and a variably adjustableslit aperture.
 13. The method of claim 9, wherein adjusting the aperturefurther comprises adjusting the adjustable slit aperture by one or bothof mounting selected one of a plurality of shims of different widths andthicknesses on the aperture, and by selectively adjusting gaps betweenthe plurality of shutters by motors.
 14. The method of claim 9, whereinpartially blocking the x-ray field with the enclosure further comprisesproviding a plurality of spokes, wherein each of the plurality of spokesare spaced apart around the periphery of the aperture wheel.
 15. Themethod of claim 9, wherein partially blocking the x-ray field with theenclosure further comprises providing each of the plurality of aperturesat an end of each of the plurality of spokes.
 16. The method of claim 9,wherein varying the amount of x-ray flux through the aperture furthercomprises moving a first pair of movable shutters and a second pair ofmoveable shutters disposed adjacent the first pair of shutters andoriented at a 90 degree angle relative to the first pair of shutters.17. The method of claim 16, wherein varying the amount of x-ray fluxthrough the aperture further comprises connecting the first pair ofmovable shutters to a first piezoelectric actuator and the second pairof movable shutters to a second piezoelectric actuator, wherein thefirst piezoelectric actuator is connected to a first voltage source andthe second piezoelectric actuator is connected to a second voltagesource.
 18. The method of claim 17, wherein varying the amount of x-rayflux through the aperture further comprises controlling the first andsecond voltage sources with a control system to move the first pair andsecond pair of shutters to vary the size of the-aperture.
 19. A methodfor producing x-ray backscatter, the method comprising: emitting anx-ray field using an x-ray tube; surrounding the x-ray tube with anenclosure, wherein the enclosure defines an end and an aperture mountedon the end of the enclosure, and wherein the x-ray field exits theenclosure through the aperture; partially blocking the x-ray field withthe aperture, wherein partially blocking the x-ray field with theenclosure further comprises providing a plurality of spokes, and whereineach of the plurality of spokes are spaced apart around a periphery ofan aperture wheel; and moving one or more shutters that are part of aplurality of shutters to vary an amount of x-ray flux through an openingof the aperture, wherein the one or more shutters move to vary at leastone of a size and a shape of the opening.
 20. A method for producingx-ray backscatter, the method comprising: emitting an x-ray field usingan x-ray tube; surrounding the x-ray tube with an enclosure, wherein theenclosure defines an end and an aperture mounted on the end of theenclosure, and wherein the x-ray field exits the enclosure through theaperture; partially blocking the x-ray field with the aperture; andmoving one or more shutters that are part of a plurality of shutters tovary an amount of x-ray flux through an opening of the aperture, whereinthe one or more shutters move to vary at least one of a size and a shapeof the opening, wherein varying the amount of x-ray flux through theaperture further comprises moving a first pair of movable shutters and asecond pair of moveable shutters disposed adjacent the first pair ofshutters and oriented at a 90 degree angle relative to the first pair ofshutters.